1. Technical Field
The present invention relates to a liquid crystal display device having an illuminating unit for illuminating a liquid crystal display panel, and more particularly to a liquid crystal display device in which an ambient light photosensor that senses external light is built into the liquid crystal display panel, and which the device controls the illuminating unit according to an output from such ambient light photosensor.
2. Related Art
Over recent years, particularly, liquid crystal display devices have been widely used not only in information and telecommunications equipment but also in electrical equipment in general. In such liquid crystal display devices, liquid crystals are non light-emitting, which means that images displayed are hard to see in dark places. Therefore these devices are provided with a backlight or a sidelight, etc., and when the external light is dim, such backlight is lit in order to illuminate the images displayed.
However, manual on/off controlling of such backlight requires the user's manipulation according to the brightness of external light. Such manipulations are bothersome, and the user sometimes ends up turning the backlight on even in bright places. In this case, using such display in a mobile telephone or similar equipment could cause the battery to run down quickly.
Accordingly, technology to address this problem has been known whereby an ambient light photosensor is built into the liquid crystal panel, the brightness of external light is sensed by such ambient light photosensor, and on/off controlling of such backlight is performed according to the sensing results (see JP-A-2002-131719 and JP-A-2000-122575).
For example, a liquid crystal display device described in JP-A-2002-131719 has a photodetector unit made of a thin film transistor (TFT) built into the liquid crystal display panel. The photodetector unit detects the photo-leakage current from the TFT ambient light photosensor and senses the brightness of external light. Thus the backlight is controlled automatically. Furthermore, a liquid crystal display device disclosed in JP-A-2000-122575 has an external light illuminance sensor and a backlight illuminance sensor, both of which employ a TFT. The backlight is controlled according to the sensing results of both sensors.
The TFT ambient light photosensors built into the liquid crystal display devices of JP-A-2002-131719 and JP-A-2000-122575 have a so-called photo-leakage characteristic that when no light is shed thereon a slight leakage current (dark current) flows in their gate-off regions, and when light is shed thereon a large leakage current flows corresponding to the intensity (brightness) of the light, as shown in FIG. 9. The TFT sensor having such characteristic is built into a photodetector circuit LS as shown in FIG. 10, for example. The photodetector circuit LS is so configured that a capacitor C is coupled in parallel between the TFT ambient light photosensor's drain electrode DL and source electrode SL, and the source electrode SL and one terminal of the capacitor C are coupled to a standard voltage supply Vs via a switching element SW. Furthermore, the TFT ambient light photosensor's drain electrode DL and the other terminal of the capacitor C are grounded GR.
The operation of the photodetector circuit LS will be described as follows. First, a constant reverse bias voltage GV (for example −10 V) is applied to the gate electrode GL of the TFT ambient light photosensor. Then the switching element SW is turned on, whereby both ends of the capacitor C are applied with a constant standard voltage Vs (for example +2 V) and charged, and the switching element SW is turned off after a predetermined time period. Accordingly, as shown in FIG. 11, a source voltage that decreases as time passes depending on the brightness of the surroundings of the TFT ambient light photosensor, that is, a charging voltage, is provided to both ends of the capacitor C. Therefore, since the charging voltage on both ends of the capacitor C is inversely proportional to the surrounding brightness of the TFT ambient light photosensor, the brightness of external light is sensed by measuring the charging voltage at a predetermined time period to after the switching element SW is turned off.
In such photodetector circuit LS, since a constant reverse bias voltage is constantly applied to the TFT ambient light photosensor's gate electrode, the state where the biased polarity voltage is always applied continues. This results in charge being trapped on the TFT gate electrode, thereby causing degradation or characteristic change in a TFT sensor element, and reduced sensitivity of the sensor. As a result, a precise photodetection cannot be performed. Accordingly, in order to prevent degradation of the sensor element due to a biased polarity, a method is known in which a reset signal is applied to the gate electrode (see JP-A-2001-169190). However, when applying such reset signal to the photodetector circuit, it is difficult to control the applying timing. Therefore, for example, when applying such signal during charging or reading to/from the ambient light photosensor, there is a risk of causing the photodetector unit and the reader to malfunction. Also, when applying at other timing, a sequence control of controlling resetting, charging, and reading becomes necessary, therefore making the control circuit configuration complicated.
In such photodetector circuit LS, during circuit operation, parasitic capacitances C1, C2 occur between the gate electrode GL and the drain electrode DL, and the gate electrode GL and the source electrode SL. When the TFT ambient light photosensor is built on a TFT substrate, these parasitic capacitances C1, C2 cannot be eliminated between the above electrodes, because of the structure of a TFT element. On the other hand, an output line (line connected to the source electrode SL) of the TFT ambient light photosensor is in a high impedance status when not charged. Accordingly, as shown in FIG. 12A, when the gate is turned on and at the moment the reverse bias voltage is converted from −10 V to +15 V, an ambient light photosensor output voltage is raised (for example, to +8 V) because of the parasitic capacitances C1, C2 (see FIG. 12C). At the same time, when the gate is turned off and at the moment the bias voltage is made to 0 V, the voltage of the drain side is lowered (for example, to −10 V) instantaneously by the parasitic capacitances C1, C2. As a result, such punch-through voltage and an inrush voltage generated by this punch-through voltage are applied to an external circuit via the output line. Accordingly, there is a risk of damaging the external circuit element coupled to the output line.
Moreover, in the case when the external light detecting circuit as described above is built into the liquid crystal display devices of JP-A-2002-131719 and JP-A-2000-122575, the photodetector circuits operate even at the time of so-called partial driving for, in general, displaying a minimum necessary part of image in the standby status of a mobile telephone, etc., when no external light is sensed. Accordingly, excess power consumption increases which causes quick battery drain.